It has been known for some time that the use and design of passive circuit elements in high quality audio amplifiers has been overlooked as a result of a preoccupation with active circuitry design. (See, for example, IAR Hotline, No. 13, September 1981, "The Sonic Importance of Passive Parts", by J. Peter Moncrieff, published by IAR, 2449 Dwight Way, Berkeley, California 94704). As a result, in many designs the benefits of improved active circuitry are not fully attained due to imperfections and distortions introduced by improper or less than optimized passive component designs. Accordingly, increasing attention has been turned to the design and use of passive elements in high quality audio amplifier designs. The present invention pertains to capacitive networks and the design of power supplies and passive signal paths in audio amplifiers.
The large energy storage requirement for power supplies in audio amplifiers makes aluminum electrolytic capacitors highly desirable as filter capacitors. A filter capacitor smooths the rectified AC voltage, stores electrical energy, and bypasses unwanted frequencies. Electrolytic capacitors, however, have more inherent inductance and other imperfections than most other types of capacitors, resulting in poor response, particularly at higher frequencies. The relatively high inherent inductance of an aluminum electrolytic capacitor limits its ability to bypass and promptly deliver fast transients of power to the audio amplifier at higher frequencies. Their other imperfections--namely, resistive losses; variation with aging, temperature, voltage, and frequency; and non-linearities due to dielectric absorption factors--introduce signal distortion which degrades the quality of sound in audio amplifier applications. These limitations are usually remedied by bypassing the electrolytic capacitor with a smaller film or tantalum capacitor that has fewer imperfections and better characteristics at higher frequencies. However, the electrolytic capacitor nonetheless negatively affects the response characteristics of the network such that its performance at higher frequencies is less optimal than it would be if the electrolytic capacitor was absent.
Aluminum electrolytic capacitors are sometimes desirable in audio amplifier signal coupling/DC blocking applications where high capacitance is required. Again, however, the poor response characteristics of electrolytic capacitors at the higher frequencies usually requires that they be used in conjunction with a smaller bypass capacitor; and, again, less than optimal performance at higher frequencies is attained due to degradation introduced by the electrolytic capacitor.